Are our minds souls? The other things being equal clause is important, for it is easy to conceive of circumstances under which a rational intuition would not constitute good evidence. For instance, if there's a drug that induces the rational intuition that P is true regardless of whether P is actually true or not, and we find that we have all taken that drug, then the fact we all get the rational intuition that P is true would not count as evidence that P is true.
Nevertheless, we could only discover that we had good reason to think we had taken such a drug (and thus discover that we had good reason to discount the relevant intuitions) on the basis of other rational intuitions. And so it is only if the principle is true that we even have the means to discover when the other things being equal clause is not met.
Yes, I agree that someone could say that about the version of the principle expressed in the argument - that is, they could deny it without affirming it. The problem, however, is that the principle they do have to affirm still has to give rational intuitions probative force and the fact they have decided only to give their own any probative force is clearly arbitrary and prejudiced. So, yes, someone could deny 1, but they could not deny 1 reasonably. That is, someone who denied 1 on the grounds that they think this principle - if and only if something appears to be the case to me is there any good evidence that it is the case - is true, is simply not a reasonable person, for they have given their own rational intuitions privileged status on an arbitrary basis. I have no problem acknowledging that wholly unreasonable people can reject a premise - they can and will reject any premise. But a reasonable person who acknowledges that their appearances have probative force is going to accept that, other things being equal, so do the appearances of others. Thus, I don't think there is any reasonable way to deny 1.
I do not understand your point about divisibility. Our reason represents our minds to be indivisible. That there are some theories about what the mind is that would, if they were true, allow the mind to be divisible, is neither here nor there. Here's such a theory - the theory that our minds are pieces of cheese. That theory - and it is good to have fancy terms for ridiculous theories, so let's call it Mental-fromagism - would allow that the mind is divisible, for cheese is clearly divisible. But so what? How's that evidence that the mind is divisible? What appearances can you appeal to that even challenge, never mind overturn, those appearances that speak in favour of premise 2?